Processes for the hydroformylation of propylene to produce butyraldehyde and butanol are well known and are practised widely. They typically employ liganded rhodium catalysed low pressure hydroformylation technology, sometimes known as the low pressure oxo (LPO) process, in which propylene is reacted with a mixture of carbon monoxide and hydrogen (known as syngas). Examples of the commercial operation of such a process are given in U.S. Pat. No. 4,247,486 and in GB-A-1,387,657.
For economic reasons, the raw material used for such industrial scale processes has been chemical grade propylene, which contains about 90-95 wt % propylene, with the majority of the balance being propane. Industrial hydroformylation processes are generally continuous processes which do not result in 100% conversation of the propylene. The selectivity to desired products is also not 100%, and a small portion of the propylene is hydrogenated to propane. Accordingly, the products of the hydroformylation of chemical grade propylene include the target butyraldehyde and butanol streams and an off gas stream containing unreacted propylene, propane and unreacted carbon monoxide and hydrogen.
It is of course economically desirable that the unreacted propylene, carbon monoxide and hydrogen be recycled. However, in order for the continuous hydroformylation process to operate successfully on an industrial scale, it is important to establish a steady state between the feed materials, including any recycle, and the degree of reaction. It is therefore important to prevent excessive propane build up in the reaction system due to the recycle of the unreacted components. However, propane and propylene are difficult to separate and thus, in order to prevent propane build up due to the recycle, it is necessary to vent off some, if not all, of the propane. This removal of propane however also involves the removal of some propylene resulting in some inefficiencies and economic debits in the process.
The present invention is concerned with the conditions that enable benefits to be realised from the use of propylene feeds of higher purity. In particular the invention is concerned with the conditions that enable benefits to be realised from the use of a propylene feedstock containing at least 97 mole % propylene, especially a propylene feedstock known as polymer grade propylene. DE-A-10035370 is concerned with an improved two reactor recycle hydroformylation system which reduces propylene losses. This recycle system is said to reduce propylene loss in the off gas. In the Example, polymer grade propylene which contains about 99.5 wt % propylene, the 0.5 wt % balance being propane, is used as the feedstock for low pressure, rhodium catalysed, hydroformylation.
The production of hydroformylated products from an olefin stream made by thermal or catalytic cracking or by dehydrogenation processes can be negatively impacted as a result of undesirable by-products coming into contact with hydroformylation catalysts. Such by-products can cause reduced efficiency in the recovery of useful hydroformylation products or can cause the formation of lower quality derivative products.
Removal of undesirable by-products from an olefin stream can be quite difficult. For example the removal of sulphur, nitrogen and chlorine from cracked hydrocarbon streams, or the removal of dimethyl ether (DME) from C4 or C5 raffinate recovered from a methyl tertiary butyl ether (MTBE) or a tertiary amyl methyl ether (TAME) unit, can require a significant amount of olefin feed pretreatment. It is, therefore, also desirable to find methods of hydroformylating olefin compositions which do not require extensive pretreatment of the olefin feed to remove contaminants.